Method and apparatus for heating glass panels

ABSTRACT

Glass panels are heated in a heating oven while supported on rolls. The glass panels are heated form above and below with convection air or with a combination of convection air and radiation heat. The convection air is heated by electric resistance elements and/or a combustible gas. The convection air passes through heat exchangers disposed in the oven en route to the glass panels.

The present application claims priority under 35 U.S.C. § 119 to patentapplication Ser. No. 20045214 filed in Finland on Jun. 9, 2004.

BACKGROUND

The present invention relates to a method for heating glass panels in aheating oven, wherein the glass panel is supported on top of rolls andsaid glass panel is heated from above and below with convection air orwith a combination of convection air and radiation heat, said convectionair being heated by electric resistance elements and/or a combustiblegas.

In addition, the present invention relates to an apparatus for heatingglass panels in a heating oven, comprising rolls for supporting a glasspanel convection blast means or a combination of convection blast meansand thermal radiators capable of heating the glass panel, and electricresistance elements or a gas burner for heating convection air.

This type of method and apparatus for heating a glass panel or sheet areprior known for example from the Applicant's patent applicationFI-20011923. In that document, disposed above and below a glass panelwithin a heating compartment are radiation heaters and convection airpipes, by which the convection air is supplied from outside the oveninto the heating compartment and blasted to the surface of a glass sheetby way of nozzles included in the convection air pipes.

The Applicant's patent application EP 721922 discloses another priorknown glass sheet heating method, based on convection blasting. Theconvection air is circulated onto the surface of a glass sheet through afan and an electric resistance element fitted in the nozzle box. An ovenapplying a similar principle is known from Patent publication EP 910553.This comprises radiation panels heated by electrical resistanceelements, the heat delivered thereby to a glass sheet providing aversatile oven configuration, regarding especially the development of atemperature profile. A principal function of the panels is theequalization of temperature differences caused by blasting at thesurface of a glass sheet.

In the process of heating glass from room temperature to a temperingtemperature of about 600-640° C., the temperature rise is consistentwith a graph 100 shown in FIG. 1. Temperature rises as a function oftime quickly at first and the rise becomes consistently slower, reachingits final tempering temperature little by little. FIG. 2 illustrates agraph 101 representing the rate of heat flow proceeding to a glass panelover the respective period. FIG. 3 illustrates the power (graph 102) ofa prior art heating source, i.e., convection-air heating electricresistance elements, as a function of time, said power correlating witha heat flow captured by glass. In convection blast systems as describedin the cited references, the electric resistance elements will have tobe rated for power outputs according to the maximum heat flow (FIG. 2)captured by a glass panel.

It is an object of the present invention to provide a method and anapparatus, enabling a glass panel to be heated more efficiently than inprior known solutions and/or convection-air heating thermal sources tobe rated for a power lower than before.

SUMMARY OF THE INVENTION

In order to achieve the above objective, a method of the invention ischaracterized in that the heating of convection air is effected by usinga heat accumulator.

Furthermore, an apparatus applying the inventive method is characterizedin that the heating oven is provided with a heat accumulator, which iscapable of heating convection air.

This solution makes it possible that, in the beginning of a heatingcycle, some of the heating effect captured by a glass panel can beclaimed from the heat accumulator which had been heated during thetreatment of a previous panel. A notable advantage of this is thatheating sources can be rated for a power lower than what is feasiblewithout a heat accumulator. A further advantage is gained by placing theheat accumulator in direct communication with heating sources which heatthe convection air (e.g., electric resistance elements or gas burners).

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described inmore detail with reference to the accompanying drawings, in which:

FIG. 1 shows, in general, the temperature of a glass panel as a functionof time in the prior art,

FIG. 2 shows, in general, the rate of a heat flow proceeding to a glasspanel, as a function of time in the prior art,

FIG. 3 shows a prior art delivery of for power from heaters, as afunction of time,

FIG. 4 shows the average heating effect as a function of time accordingto the invention, and

FIG. 5 shows one embodiment of an apparatus applying the inventivemethod.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

One exemplary embodiment for an apparatus applying the inventive methodis shown in FIG. 5. The apparatus is a heating oven 1, inside whosewalls 2 is provided a compartment 2 a to be heated. A glass panel 4 isbrought into the compartment 2 a to be heated on a roll conveyor 3constituting a substantially horizontal conveying track. The compartment2 a to be heated is provided with upper blast means 5 a, 5 b, 11 andlower blast means 6 a, 6 b, 12 for convection air. These includepreferably ducts 5 a, 5 b and 6 a, 6 b, the horizontal duct sections orboxes 5 b and 6 b thereof being provided with nozzles for blasting air Ato top and bottom surfaces of the glass panel 4. The blasting power ofair A can be adjusted for example by means of fans 11 and 12 disposed incommunication with the duct sections 5 b and 6 b. The oven 1 can befurther provided with conventional radiation heaters (not shown),capable of heating a glass panel directly. The radiation heaters aremounted preferably above and below the glass panel 4, for examplealongside the blast means.

In communication with duct sections 5 a and 6 a, on a suction side ofthe fans 1 1 and 12, are disposed heat accumulators 9 and 10 accordingto the present invention. Each heat accumulator comprises preferably agenerally solid, but porous body, manufactured preferably of a heataccumulating material, such as metal, ceramics, silicon carbide orstone. The accumulators 9 and 10 define their own internal passages orflow paths, whereby the convection air A is adapted to proceed throughthe heat accumulators 9 and 10. The hot air A, blasted onto the glasspanel's 4 surface, is circulated within the compartment 2 a.Accordingly, the air A, blasted onto the glass panel 4, is guided(sucked) primarily from the glass panel 4 back to the heat accumulators9 and 10. The accumulators 9 and 10 deliver heat, thus heating the air Apassing through the accumulators.

Preferably, the means for heating the convection air (electricresistance elements and/or gas burners) are placed in directcommunication with the accumulators. Thus, as shown, disposed in directcommunication with the heat accumulators 9 and 10 are disposedrespective electrical resistance elements 7 and 8 used for heating theheat accumulators 9 and 10. The electrical resistance elements can bereplaced or supplemented for example with gas burners, the heat of whichis generated by a combustible gas. Unlike the prior art, a primaryfunction of the electrical resistance elements 7 and 8 is heating theheat accumulators 9 and 10, whereby the electrical resistance elementscan be rated for top power outputs which are lower than the heatingeffect needed at the early stage of heating the glass panel 4.Furthermore, the electrical resistance elements can be optimized forsuch a power that the power output delivered thereby is substantiallyunchanged throughout the heating cycle. This unchanged power output,i.e., the average heating effect, is represented by a graph 103 shown inFIG. 4. In practice, of course, the electrical resistance elements 7 and8 may have a power output hovering anywhere between the graphs 102 and103, yet preferably closer to the graph 103. The achievable proximity tothe graph 103 depends on the solidity of the heat accumulators 9 and 10and the efficiency of heat transfer (heat transfer area) between theheat accumulators 9 and 10 and the convection air A. An example will nowbe described regarding such operation of me inventive apparatus.

At the initial stage of heating, a cold glass panel 4 is heated by meansof the heat accumulators 9 and 10 or by a combined action of the heataccumulators 9 and 10 and the electrical resistance elements 7 and 8.Heat is delivered thereby to convection air A to be recirculated with apower which substantially matches the graph 101 of FIG. 2. Theelectrical resistance elements 7 and 8 strive to heat the heataccumulators 9 and 10 simultaneously with a given substantiallyunchanging power.

At the initial stage, the electrical resistance elements 7 and 8 neednot provide a power sufficiently high to maintain the initialtemperature of the heat accumulators 9 and 10 which have been heatedduring the treatment of a previous glass panel. When the heat flowproceeding to the glass panel 4 begins to decline over the final stageof heating, as depicted in FIG. 2, the power of the electricalresistance elements 7 and 8 reaches a limit at which some of the powerdelivered thereby is sufficient for heating the heat accumulators 9 and10 and some of the power delivered thereby Is sufficient for generatinga heat flow required by the glass panel 4 at the final stage of heating(in other words, for heating the convection air passing through the heataccumulator). This is in part enabled by recirculation of the convectionair, the heating of which, especially at the final stage, only requiresa small amount of power generated by the electrical resistance elements.This way, the heat accumulators can be heated to their initialtemperature while completing the heating of a glass panel.

Furthermore, the electrical resistance elements 7 and 8 need not beadjusted during a heating cycle with respect to their power outputs orthe adjustment demand is essentially lesser than in prior art solutions.In addition, the electrical resistance elements need not be rated tomatch a peak power output, which is needed for generating a high-powerheat flow for the initial stage of heating the glass panel 4.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, modifications, substitutions, and deletionsnot specifically described may be made without departing from the spiritand scope of the invention as defined in the appended claims.

1. A method for heating glass panels supported on rolls in a heatingoven, comprising the steps of: A) directing convection air to the glasspanels from above and below the glass panels; B) heating the convectionair by a heat source; and C) passing the convection air in heat exchangerelationship with a heat accumulator disposed in the oven.
 2. The methodaccording to claim 1 wherein step B is performed by a heat sourcecomprising an electric resistance element.
 3. The method according toclaim 1 wherein step B is performed by a heat source comprising a gasburner.
 4. The method according to claim 1 wherein the heat accumulatoris directly heated by the heat source.
 5. The method according to claim4 wherein the heat source comprises an electric resistance element. 6.The method according to claim 4 wherein the heat source comprises a gasburner.
 7. The method according to claim 1 wherein convection airdirected to the glass panels from above, flows through a first heatexchanger, and convection air directed to the glass panels from belowflows through a separate heat exchanger.
 8. The method according toclaim 1 wherein during an initial stage of heating of a glass panel theconvection air is at least partially heated by residual heat from theheat accumulator, wherein the heat source operates at a maximum powerrate less than heating power required during the initial stage ofheating the glass panel.
 9. The method according to claim 1 wherein thepower source operates at a substantially constant power.
 10. The methodaccording to claim 1 wherein step A further includes heating the glasspanels by radiant heat.
 11. The method according to claim 1 wherein stepC comprises passing the convection air through a porous heat exchangerbody.
 12. An oven for heating glass panels comprising: a heatingchamber; rolls in the heating chamber for supporting glass panels; airdirecting structure in the heating chamber for directing convection airagainst the glass panels from above and below the glass panels; a heatsource for heating the convection air; and a heat accumulator disposedin the heating chamber and arranged wherein the convection air passes inheat-exchange relationship therewith.
 13. The oven according to claim 12wherein the heat source comprises an electric resistance element. 14.The oven according to claim 12 wherein the heat source comprises a gasburner.
 15. The oven according to claim 12 wherein the heat source isarranged to directly heat the heat exchanger.
 16. The oven according toclaim 15 wherein the heat source comprises an electric resistanceelement.
 17. The oven according to claim 15 wherein the heat sourcecomprises a gas burner.
 18. The oven according to claim 1 wherein afirst heat accumulator heats convection air directed against the glasspanels from above, and a separate heat accumulator heats convection airdirected against the glass panels from below.
 19. The oven according toclaim 12 further including radiant heaters in the heating chamber forheating the glass panels by radiant heat.
 20. The oven according toclaim 12 wherein the heat accumulator comprises a porous body throughwhich convection air flows.